There are limitations on the positions and the number of cameras mounted on a vehicle. In order to eliminate blind spots in image capture to the greatest extent possible under such limitations, vehicles are provided with wide-angle lenses, such as fisheye lenses, that can capture the widest possible range of the vehicles' surroundings at once. However, in an image captured using a wide-angle lens, such as a fisheye lens, a subject is distorted in a circular shape, and the distortion intensifies toward an outer edge of the captured image. Such lens distortion can be resolved by correction conforming to correction parameters suited for the lens characteristics. However, application of intense distortion correction unfavorably makes invisible the surrounding areas that have been advantageously captured.
There is a demand for an image for monitoring the surroundings of a vehicle at the time of, for example, parallel parking and garage parking. It is desired that such an image shows edges of a vehicle body in real space and straight lines parallel to these edges of the vehicle body while maintaining their original parallelism to the greatest extent possible. In order to address this desire, Patent Document 1 describes a periphery monitoring device that applies distortion correction processing to a captured image including edges of a vehicle body such that parallel perception is similar to that in real space, and displays the resultant image on a monitor. More specifically, this periphery monitoring device includes: a coordinate axes setting unit for setting, in an image captured by a camera, a first axis along an edge of a vehicle body and a second axis orthogonal to the first axis, the camera being disposed so as to include the edge of the vehicle body in the captured image; and a distortion correction processor for enlarging the captured image at a magnification ratio that increases in the second axis direction in a nonlinear fashion that depends on the distance from the second axis while the first axis direction is kept substantially fixed, wherein the captured image that has been corrected for distortion by the distortion correction processor is displayed on a display device.
However, distortion correction processing of this periphery monitoring device is modification in which horizontal pixel rows composing the captured image are merely enlarged at different magnification ratios in accordance with their positions in the vertical direction. Therefore, this is insufficient as lens distortion correction for an ultra wide-angle lens, such as a fisheye lens. Furthermore, correction parameters are magnification ratios that are uniquely determined by the positions in the vertical direction. As horizontal pixel rows are merely enlarged in accordance with these magnification ratios, there is a possibility that desired distortion correction cannot be applied.